<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box{
        width: 0px;
        height: 0px;
        border: 200px solid;
        /* border-color: red green skyblue yellow; */
        border-color: transparent transparent skyblue transparent;
    }
    /* 伪类：用来辅助选择的标签或者类名 */
    /* after在。。。之后 */
    .con::after{
        content: '123';
        font-size: 20px;
        color: red;


    }
    /* before在。。。之前 */
    .con::before{
        content: 'yuioy';
        font-size: 30px;
        color: blue;
    }

    .box1::after{
      display: inline-block;
      content: '';
      border: 30px solid transparent;
      border-bottom-color:red ;
    }
</style>
<body>
    <div class="box"></div>
    <!-- 伪类 :nth-of-child -->
    <div class="con">我是伪类的内容</div>

    <div class="box1"></div>
</body>

</html>